Research On Operation Control Strategy Of Synchronous Fixed Frequency Microgrid Based On U-I Drooping

Microgrid is an effective way to use distributed renewable energy efficiently.The combination of microgrid and large grid is the future development direction of power system.The research object in this paper is the low-voltage microgrid,and the low-voltage distribution line exhibits high R/X characteristics,which leads to the active power and reactive coupling of distributed power sources in the micro-grid with traditional droop control.It is difficult to reasonably distribute the power according to their respective capacities.At the same time,the asynchronous control between the micro-source inverters makes the control strategy complicated and difficult to control.Abandoning the constraints of traditional microgrid control strategy,this paper proposes a synchronous fixed-frequency microgrid control strategy based on U-I droop,which uses synchronous clock or phase information provided by synchronous phasor measurement(PMU)device as the global synchronization reference of each inverter.Control all inverters in the microgrid to run synchronously,and control the inverter output power by using the U-I droop control relationship.The inverters in the micro grid are operated similarly to a single inverter,thereby simplifying the correspondence between power and voltage frequency and reducing the difficulty of microgrid control.This paper firstly describes the basic principle of the traditional droop control strategy,and points out the existing shortcomings of the micro-grid operation in the traditional droop control strategy,which is greatly affected by the line operating conditions and the uneven power distribution among the inverters.Aiming at the shortcomings of the traditional droop control strategy,a multi-inverter microgrid control strategy based on U-I droop is proposed.The voltage-current sag relationship(U_d-I_d)of the d-axis in dq coordinates controls the active power flow of the inverter,and the voltage-current sag relationship(U_q-I_q)of the q-axis controls the reactive power flow of the inverter to achieve active power and no decoupling control of power.By using the controllable frequency of the inverter,the frequency of each DG inverter is controlled to be fixed,and the problem of non-synchronization between the inverters in the traditional droop control strategy is solved,and the control difficulty of the underlying inverter is reduced.The operation characteristics of the control strategy under different line conditions indicate that the proposed control strategy is not constrained by line conditions.According to the proposed new control strategy block diagram,the droop controller suitable for synchronous fixed-frequency microgrid is designed.Finally,in order to verify the validity of the proposed theory,this paper builds a microgrid simulation model containing three DGs based on the control theory using Matlab/Simulink simulation software.The simulation results show that the proposed control strategy can effectively realize the active and reactive decoupling control,so that the load can be properly distributed among different capacities DG,and the plug-and-play of the DG unit is guaranteed,and the power quality is improved.In order to further verify the correctness of the proposed theory,the micro-grid experimental platform is used to verify the U-I drooping synchronous fixed-frequency microgrid control strategy.The experimental results show that the control theory proposed in this paper is feasible in practical engineering applications and lays a solid foundation for further engineering application.